The present invention relates in general to wireless communication systems, such as but not limited to cellular communication systems, and is particularly directed to a scheme for increasing the capacity of broadband base station without a significant increase in hardware, by combining a set of wideband digital radios with a phased array antenna to provide higher channel reuse and higher trunking efficiency.
A conventional xe2x80x98omnixe2x80x99 broadband personal communication system (PCS)/cellular base station, which employs a wideband digital transceiver and associated omnidirectional antennas, cannot provide high capacity in a multiple base station environment. Due to the presence of co-channel interferers, the base station either has no neighbors or has a high channel reuse factor (e.g., K=11 or greater). Unfortunately, a high reuse factor means that only a small fraction (e.g., on the order of one-tenth) of the available power of the receiver is being used.
As a consequence, an omni base station suffers a significant cost disadvantage compared with those that employ narrowband systems operating at only a limited number of channels, since receiver cost is the same regardless of whether it is being fully utilized. For example, for a 5 MHz PCS GSM system, twenty-four RF channels having eight voice channels per RF channel provides 192 total channels. At a frequency reuse factor of K=11, two RF channels at eight voice channels/RF channel provides sixteen available channels, resulting in an Erlangs/base station at a grade of service of 0.02 equal to 9.83.
One way to increase capacity is to implement a sectorized wideband base station employing directional antennas to subdivide the spatial coverage (e.g., into three 120xc2x0 sectors). Although reducing the number of potential interferers, this approach suffers from reduced channel use (e.g., K=4, which allows use of only one-fourth of the available channels). In addition, a sectorized wideband radio suffers trunking efficiency loss. For example, for the above 5 Mhz PCS GSM example, at K=4, six RF channels at eight voice channels per RF channel yields 48 total channels, or 16 channels per sector for three 120xc2x0 spatial sectors. At an Erlangs/sector of 9.83, the resulting Erlangs/base station is somewhat improved over that of a conventional omni base station at (9.83xc3x973) 29.49 Erlangs.
In accordance with the present invention, a high capacity broadband base station may be realized without a significant increase in hardware, by combining wideband digital radio equipment with a phased array antenna, so as to provide dynamic beam steering via the phased array. As a result, the capacity of the base station is increased significantly.
For this purpose, the phased array broadband PCS/cellular base station- architecture of the present invention comprises a phased array antenna subsystem coupled to a wideband radio and array processing subsystem. The phased array antenna subsystem contains multiple sets or pairs of alternating receive only and transmit/receive elements distributed in a two dimensional spatial array. Each transmit/receive antenna element is coupled through an associated diplexer to wideband receiverxe2x80x94transmitter pair. Each receive only antenna element is coupled to an associated wideband receiver. The receivers"" RF outputs are downconverted via a broadband downconverter to intermediate frequency signals. The input to each transmitter is coupled to the output of a broadband IF-RF upconverter.
The IF output signals of the respective downconverters are digitized for application to downstream digital signal processing in an associated-wideband digital radio. In the transmit direction, digital IF signals from the wideband digital radio are converted into analog format. Each digital wideband radio performs both receive and transmit channel signal processing. In the receive direction, the digital representation of the entire spectrum for each antenna element is divided into channels for the particular waveform of interest. For a 5 MHz PCS GSM, the digital wideband radio separates twenty-four carriers into twenty-four (200 KHz wide) data streams, each of which is representative of a respective channel, and couples each channel to a digital signal processor (DSP). In the transmit direction, the radio combines the digital representations of the twenty-four individual channels supplied by the DSP into a single wideband channel for transmission.
The array processing modules employed by the DSP include a set of weighting coefficient multipliers, whose respective (amplitude and phase) weighting coefficients are periodically updated by a receiver weighting coefficient calculation algorithm. The resulting weight vector is multiplied by the received signal components and summed to provide a composite signal to a downstream demodulator. The demodulator output feeds a vocoder, with which voice signals are bidirectionally interfaced.
In the transmit direction the DSP receives the serial data stream supplied by a modulator, which receives its input from the vocoder. For outgoing signals, the DSP and executes a transmit array processing module, which contains a set of weighting coefficient multipliers. These transmit weighting coefficient multipliers are coupled to divide channels from a signal splitter to which the output of modulator is applied. The transmit weighting coefficients may be derived from the received steering vector using a transformation operator. The outputs of the multipliers constitute the respective channels to be transmitted.
For the above example of a 5 Mhz PCS GSM system, the phased-array broadband base station system of the invention reduces co-channel interference substantially, allowing higher frequency reuse. A reuse factor of K=3 allows eight RF channels times eight voice channels per RF channel, for a total of sixty-four channels. This results in an Erlangs/base station at a grade of service of 0.02 equal to 53.4.